Only the physical size differs. The AXY are cast in the same shell as the original Series I pickups. When we started to make smaller body shapes like the Essence, we needed to make the pickup as small as possible, so the MXY was made.

Internally, the pickups are identical. If you've got AXYs in your bass, you can use FatBoy pickups, which are the same physical size as the AXY, but the coil is wound on the larger Series I magnet.